实现单例模式

1. 只适用单线程

class Singleton {

private static Singleton instance;

private Singleton() {

}

public static Singleton getInstance() {

if(instance == null) {

instance = new Singleton();

}

return instance;

}

}

2. 多线程

class Singleton {

private static Singleton instance;

private Singleton() {

}

public synchronized static Singleton getInstance() {

if(instance == null) {

instance = new Singleton();

}

return instance;

}

}

3. 类内加载

class Singleton {

private static Singleton instance = new Singleton();

private Singleton() {

}

public  static Singleton getInstance() {

return instance;

}

}

4. 双重校验锁

class Singleton {

private static Singleton instance;

private Singleton() {

}

public  static Singleton getInstance() {

if(instance ==null) {

synchronized(Singleton.class) {

if(instance == null) {

instance = new Singleton();

}

}

}

return instance;

}

}